1. Field of the Invention
This invention relates to an information signal transmission system of the kind arranged to transmit information signals by compressing them.
2. Description of the Related Art
The conventional information compressing transmission methods include, for example, a method called the time axis transform method (hereinafter referred to as the TAT method). In the TAT method, an information signal is compressed for transmission by utilizing the fact that, in compressing the band-width of an information signal, the density of the information signal in one part thereof differs from another.
FIG. 1 of the accompanying drawings shows the operating principle of the TAT method as applied to a case where a signal is processed in a one-dimensional manner. Referring to FIG. 1, as indicated with broken lines, an original signal is divided into groups or blocks of picture elements by a given account of information. Each of the divided groups is subjected to a discrimination between a dense state and a sparse state of information. For a group which is thus determined to be dense, all data that are obtained by sampling the original signal are transmitted as transmission data. Meanwhile, in the case of a group determined to be sparse, only a part of data obtained by sampling is transmitted while the remainder of the same group is not transmitted and regarded as skipped data. In the illustration, marks " " represent the transmitted picture element data or data to be transmitted. Marks "X" represent data not transmitted (or skipped data). The band of the transmitted information signal is compressed as the number of data transmitted per unit time is decreased by the arrangement of transmitting the data marked " " at intervals of a given length of time.
With the picture element data thus transmitted, the skipped picture elements which are not transmitted are proximately restored by using the transmitted data to obtain interpolating data which are as indicated with marks " " in FIG. 1. The interpolating data thus obtained corresponds to the sparse parts of information and has a close resembrance to the skipped data. Therefore, the signal thus restored virtually shows no difference from the original information signal, in comparison with a case where all the data are transmitted, despite of a great degree of compression effected on the transmitting band of the information signal.
In this instance, each of the data (or picture element) groups is examined to determine whether all the picture elements within the group is to be transmitted or only a part of the picture elements included in the group is to be transmitted. This determination is made by checking the fineness or elaborateness of the original signal. Information on this determination is transmitted as a transmitting mode signal concurrently with the information signal.
Further, in the case of image information signal, the transmitting band of the image information can be compressed by changing not only the horizontal sampling intervals but also vertical sampling intervals in a two-dimensional manner.
In two-dimensionally processing a signal, like in the case of an image information signal, one image plane is divided into blocks, each having an m.times.n number of picture elements. Then, the image density of each block is examined. A block which is determined to be dense is processed to have all the picture elements included in the block sampled and transmitted as transmission data. A block determined to be sparse is processed to have only some of the picture elements thereof sampled and transmitted as transmission data while the rest are not transmitted and processed as skipped data.
Assuming that the processing or transmitting mode in which all the picture elements are to be sampled is called "mode E" and the mode in which only a part of picture elements are to be sampled is called "mode C", the picture elements to be transmitted and the picture elements not to be transmitted in each of these different modes E and C are as shown in FIGS. 2(a) and 2(b).
FIGS. 2(a) and 2(b) show the picture element blocks, each consisting of 4.times.4 picture elements. The block shown in FIG. 2(a) is to be processed in the mode E while that of FIG. 2(b) to be processed in the mode C. The image information of one image plane to be transmitted is divided into blocks, each having 4.times.4 picture elements, from a left upper part to a right lower part of the image plane one after another. One of the above-stated two different transmitting modes is selected for each of these blocks according to the density of image carried. Sampling is performed in the mode selected.
FIG. 3(a) shows one field of TV image plane of the NTSC system divided by the above-stated method into picture element blocks, each of which consists of 4.times.4 picture elements. The transmitting modes E and C which are as shown in FIG. 2(a) and 2(b) are allocated to these divided blocks as applicable. Marks " " indicate picture elements to be transmitted and marks "X" picture elements not transmitted. With the picture elements sampled in this manner, the transmitting band of the information signal is compressed by transmitting at given intervals the data thus obtained through sampling.
The transmission of the image data sampled from within each block is arranged to be effected one after another in sequence either in the horizontal direction or in the vertical direction. FIG. 3(b) shows a case where these data are transmitted one after another in the horizontal order. FIG. 3(c) shows a case where the transmission is effected in the vertical order. The reference symbols and numerals in FIGS. 3(b) and 3(c) correspond to those used for the picture elements shown in FIG. 3(a). These data are transmitted in the direction of arrows.
The picture elements which are not transmitted are proximately restored during reproduction by using the adjacent picture elements which are transmitted. Therefore, despite of the great extent of compression effected on the transmitting band of the information signal, there takes place not much changes in the amount of information as compared with a case where all the data are arranged to be transmitted.
However, there arises some difference in information level among the picture element because of positional relation, on the image plane, between the picture elements before and after transmission change-over from one block to another. For example, picture elements a and 16 which are neighboring each other across a border line between two blocks as shown in FIGS. 3(b) and 3(c) are separated as shown in FIG. 3(a) by one picture element in the horizontal direction and by four picture elements in the vertical direction. Therefore, their correlativity becomes weak, particularly in the vertical direction. This results in information level difference between one picture element and another. In other words, the probability of having a high frequency component at a boundary between one block and another becomes high. In the case of the two-dimensional TAT system in particular, there are a large number of blocks. Therefore, the probability of having a high frequency component at the boundary between adjacent blocks further increases. An image signal converted by the the two-dimensional TAT method, therefore, tends to have as a whole many high frequency components included therein.
In transmitting such an image signal, the high frequency component thereof greatly deteriorates if the transmissible band of the transmission line is narrow.
This tends to result in transmission errors and generation of noises. That trouble may be avoidable by widening the transmissible band of the transmission line. Then, however, it increases the cost of the transmission system.
Further, in the conventional information compressing transmission system, only a part of picture elements within some block to be processed in the mode C are sampled and transmitted as information data. In decoding the whole picture element information data of the block thus transmitted, an interpolating process is performed by using transmitted picture element information data for the non-transmitted picture elements. It has been inevitable, therefore, to have a certain degree of deterioration of the information. In the case of a fine image information signal in particular, it has been necessary to carry out a complex process of interpolation during a decoding operation for the purpose of minimizing the deterioration of information because of the above-stated poor correlativity among the picture elements sampled for transmission.